Cytokinins are a class of N6 substituted purine derivative plant hormones that regulate cell division, as well as a large number of developmental events, such as shoot development, root branching, control of apical dominance in the shoot, leaf development, chloroplast development, and leaf senescence (Mok, et al., (1994) Cytokinins. Chemistry, Action and Function CRC Press, Boca Raton, Fla., pp. 155-166). Active cytokinin pools are regulated by the rate of synthesis, storage, and/or degradation. In maize, cytokinins were found to play a role in establishing seed size, decreasing tip kernel abortion, and increasing seed set during unfavorable environmental conditions (Cheikh and Jones, (1994) Plant Physiol 106:45-51; Dietrich and Morris, (1995) Plant Physiol Biochem 33(5):327-336).
The irreversible degradation of cytokinins, catalyzed by cytokinin oxidase, is an important mechanism by which plants modulate their cytokinin levels (Houba-Herin, (1999) Plant Journal 17:615-626; Morris, et al., (1999) Biochemical and Biophysical Research Communications 255:328-333; Brugière, et al., (2003) Plant Physiol 132:1228-1240). The catabolic enzyme cytokinin oxidase (CKX) plays a major role in controlling cytokinin levels in plant tissues, and CKX activity has been found in a great number of plant tissues. The CKX enzyme is a FAD-containing oxidoreductase that catalyzes the degradation of cytokinins bearing unsaturated isoprenoid side chains. The CKX enzymes irreversibly inactivate most cytokinins by cleaving the isoprenoid side chain from the adenine ring (Armstrong, et al., (1994) Cytokinins. Chemistry, Action and Function. CRC Press, Boca Raton, Fla., pp. 139-154).
It was earlier shown that ZmCkx1 gene expression is inducible in various organs by synthetic and natural cytokinins. ZmCkx1 is also induced by abscisic acid, which may control cytokinin oxidase expression in the kernel under abiotic stress. Under non-stress conditions, cytokinin oxidase in maize may play a role in controlling growth and development via regulation of cytokinin levels transiting in the xylem. Under environmental stress conditions, cytokinin oxidase gene induction by abscisic acid results in aberrant degradation of cytokinins, therefore impairing normal development (Brugière, et al., 2003, supra).
In view of the influence of cytokinins on a wide variety of plant developmental processes, including root architecture, shoot and leaf development, and seed set, the ability to manipulate cytokinin levels in higher plant cells, and thereby affect plant growth and productivity, is of great commercial value.